The invention relates to a traction mechanism, particularly for driving camshafts of an internal-combustion engine, including a driving wheel, an output wheel, a traction device, as well as a first and a second tensioner rail.
The driving of camshafts of internal-combustion engines normally takes place starting from the crankshaft by way of a traction device, such as a timing chain or timing belt. Tensioners for tensioning the traction device are used for preventing transverse vibrations, in particular. Such an arrangement having a chain tensioner including a saddle, which is mechanically acted upon by the force of a leaf spring, is described in German Patent document DE 4114948 A1.
When the tension is increased in the side of the belt assigned to the tensioner, for example, as a result of oscillations of the camshafts, a play occurs at least for a short time in the other end, in an arrangement described, for example, in German Patent document DE 4114948 A1, so that undesirable transverse vibrations are incited in the traction device. A driving system as illustrated in German Patent document DE 201 02 748 U1 could provide a remedy, according to which two tensioner rails are swivellably linked on one side to a housing of a tensioning device, and hydraulic pistons with an opposite tensioning direction which act from the inside against the tensioner rails are arranged in the housing between the tensioner rails. The pistons are supplied by way of a common hydraulic connection, so that the tensioning action is coordinated. The driving system from German Patent document DE 201 02 748 U1 comprises two tensioning devices which, including the housing, the tensioning pistons and the tensioner rails, are constructed symmetrically with respect to a center line.
According to German Patent document DE 201 02 748 U1, the two swivellable tensioner rails are mutually connected by way of the two coupled hydraulic pistons in a force- and motion-transmitting manner. However, there is neither a reference to the fixed internal-combustion engine housing, nor can the transmission of vibrations of one tensioner rail, for example, incited by oscillating camshafts, to the other tensioner rail, be controlled. Furthermore, the symmetrical development of the tensioning device, which causes a linear vibration system, promotes the buildup of vibrations and thus the risk of damage to, or even the destruction of, the traction mechanism. Air trapped in the hydraulic system of the pistons may present another problem, which has the result that an unknown spring system exists between the tensioner rails.
It is therefore an object of the invention to provide a traction mechanism which permits a defined and controlled action of tension force upon the tensioner rails for the effective prevention of undesirable transverse vibrations. In addition, the traction mechanism is to have a simple and advantageous construction and should be producible, mountable and maintainable.
This object is achieved according to the invention, in which a tensioner rail can be acted upon by tension force by way of a first tensioning device supported on one side on the tensioner rail and, on the other side externally on a fixed reference point, such as the internal-combustion engine housing. The first tensioner rail is connected with the second tensioner rail in a defined force- and motion-transmitting manner. In this fashion, the vibration of a tensioner rail can be controlled with respect to a fixed external reference point and no undefined or undamped vibration transmission can take place from one tensioner rail to the other. The essential parameters of the traction mechanism are defined and can be coordinated in a favorable manner.
On the whole, the vibrations of the camshaft drive and the loading of the traction device are considerably reduced by the solution according to the invention. The traction device may be constructed to be significantly lighter and the operational reliability can clearly be increased. The operation of the charge cycle valves of the internal-combustion engine by way of the cams can be better controlled resulting in a more precise timing, so that the internal-combustion engine may be operated more efficiently. The improved camshaft-side traction mechanism also has positive effects on the drive of the auxiliaries, such as the water pump, the air-conditioning compressor, the starter and/or the generator, in that these auxiliaries may now also have a lighter construction or their operational reliability and service live is increased. Finally, as a result of a reduction of the crankcase vibrations, noise generating is reduced; the timing gear runs more smoothly. The reduction of vibrations finally benefits the entire transmission line.
Particularly preferred embodiments and further developments of the invention are described and claimed herein.
It was found to be advantageous for the tensioner rails to be connected by way of a spring-and-damper device. The transmission of vibrations from one tensioner rail to the other tensioner rail therefore takes place as a function of the parameterization of the spring-and-damper device in a sprung and/or damped manner, a coordination of the spring fraction and of the damper fraction being permitted within largely arbitrary limits corresponding to the system requirements. According to a very expedient embodiment, the spring-and-damper device has a fixed characteristic curve, in which case a particularly advantageous characteristic curve can be determined by tests and/or calculation. Cost-effective standard components can be used.
According to another embodiment, the spring-and-damper device is adjustable. The characteristic curve of the spring-and-damper device can then, for example, be individually coordinated with the system within the framework of the assembly and, as required, can be readjusted after some time during the operation and can be adapted to changed marginal conditions.
According to another embodiment of the traction mechanism according to the invention, the tensioner rails are connected by way of a two-part joint device, a first part of the joint device being assigned to the first tensioner rail and a second part of the joint device being assigned to the second tensioner rail. The two parts of the joint arrangement, which are rigid per se, can be moved with respect to one another and thus the two tensioner rails can also be moved with respect to one another in a defined manner. An intervention into the joint arrangement permits a control of the movement.
It is particularly preferable for the second part of the joint device, starting from a pivot assigned to the second tensioner rail, to have two lever arms, in which case a first lever arm is connected with the first part of the joint device, and a second lever arm can be acted upon by tension force by a second tensioning device. The force applied by the second tensioning device acts by way of the second lever arm; a rotatory action upon the second part of the joint device takes place. By way of the first lever arm, the first part of the joint device is pulled toward the second part of the joint device, so that the traction device is tensioned.
It is a significant advantage for the second tensioning device to be supported on the second lever arm on the one hand, and, on the internal-combustion engine side, on the other hand. In this manner, the vibration of the second tensioner rail with respect to the internal-combustion engine housing, which represents a fixed external reference point, can be controlled, and the two tensioner rails cannot vibrate in an undefined or undamped manner.
It is very expedient for the axes of the lever arms to be arranged at an angle with respect to one another starting from the pivot. The size of the angle depends on the construction and may be in the range of, for example, 100° to 130°.
It was found to be advantageous for the first part of the joint device to have an essentially straight further development, one end being connected with the second part of the joint device and the other end being connected with the first tensioner rail in a force- and motion-transmitting manner. A straight further development of the first part of the joint device is advantageous under the aspect of a predominant tensile stress but a different further development may be preferred, possibly because of marginal constructive conditions.
It is also particularly preferable for the first part of the joint device to have two lever arms starting from a pivot assigned to the first tensioner rail, in which case one lever arm is connected with the second part of the joint device and the other lever arm can be acted upon by tension force by way of the first tensioning device. In this case, an additional tensioning device will not be necessary, so that the traction mechanism as a whole consists of fewer parts. By way of the first tensioning device, which is externally supported on a fixed reference point, the first tensioner rail can be acted upon in a defined and controlled manner and, simultaneously, the first and the second tensioner rail can be acted upon in the tensioning direction in a defined and controlled manner by way of the two-part joint device.
Even if only one tensioning device is used, it is just as expedient as when using two tensioning devices for the axes of the lever arms to be arranged at an angle with respect to one another starting from the pivot; for the size of the angle to be dependent on the construction, for example, within the range of from 120° to 140°; and for he first part of the joint device to have an essentially straight further development. In that case, one end is connected with the second part of the joint device and the other end is connected with the first tensioner rail in a force- and motion-transmitting manner.
In another, also preferable embodiment of the traction mechanism according to the invention, the two parts of the joint device are mutually connected in a hinge point, and the hinge point can be acted upon by tension force by a second tensioning device. This embodiment does not require a part of the joint device which is acted upon rotatorily, and therefore has a kinematically simpler construction. The coordination of the tensioning action is, therefore, also correspondingly simpler.
The second tensioning device is expediently supported on the hinge point, on the one hand, and on the internal-combustion engine side, on the other hand, so that a defined and controlled tensioning of the traction device becomes possible with respect to the fixed housing of the internal-combustion engine.
According to another, also preferable embodiment of the invention, a second tensioning device acts between the tensioner rails, which, on one side, is supported on the first tensioner rail and, on the other side, is supported on the second tensioner rail. Even if, in this case, a reference to a fixed external reference point takes place only by way of the first tensioning device which is assigned to the first tensioner rail, a defined and controlled tensioning of the traction device can also be achieved by way of the second tensioner rail by way of the second tensioning device.
It is very advantageous for the second tensioning device to, on the one side, be connected directly with a tensioner rail, to be arranged outside the tensioner rails and to, on the other side, be connected with the other tensioner rail by use of a connection element. For constructive reasons, it is advantageous to further develop the connection element in the shape of a crescent.
A hydraulic, mechanical, electromagnetic, electromotive or pneumatic operation of the first and/or second tensioning device is particularly preferred. The tensioning device expediently includes a piston which can be acted upon hydraulically, in which case the tensioning device can be hydraulically supplied with lubricating oil of the internal-combustion.
It may be considered to be very advantageous if the tensioning device can be acted upon by an at least approximately constant pressure, which does not fall below a predetermined minimum value, so that the traction device can be constantly tensioned by a predetermined minimum force. According to another, also advantageous embodiment, the tensioning device can be acted upon in a controlled manner by a discretely or continuously varying pressure, so that the tension of the traction device can be adapted to changing marginal conditions.
According to a particularly preferred embodiment of the invention, the first tensioning device and/or the second tensioning device may be supplied with pressure medium by way of a check valve. In this manner, an at least one-sided uncoupling from the pressure medium supply is achieved, so that, for example, both tensioning devices can be supplied with oil from the oil circulating system of the internal-combustion engine, but vibrations of one tensioner rail nevertheless have no negative influence on the other tensioner rail. The pressure medium acts upon the first and/or the second tensioner rail in the tensioning direction; the check valve ensures that the tensioning pressure is maintained.
It is very expedient for the first and the second tensioner rail to have differently shaped guiding areas, so that the traction mechanism does not have a linear construction and the incitation or continuation of vibrations is avoided from the start.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.